Fresnel lenses include a series of optical facets, wherein each optical facet is separated by a riser. That construction provides for a substantially planar lens useful in many different applications.
One application in which fresnel lenses are particularly useful is in overhead projectors where they help to focus light from beneath the stage and onto the mirror elevated above the stage. In that application, however, light refracted through the risers in the fresnel lens causes what is commonly referred to as "stage glare." Stage glare typically affects the operator of the overhead projector by disrupting their view of the materials on the stage of the projector.
Attempts to reduce stage glare have involved providing a diffuse surface on the risers to diffuse the light refracted through those surfaces, thereby reducing the glare. The diffuse risers have been provided by chemically etching or attacking the finished lenses or the dies used to form the lenses. Typically the entire lens/die is treated, including the optical facets as well as the risers. After etching, the optical facets are recut or otherwise processed to restore them to a smooth, specular finish while the risers remain etched to provide the desired diffuse surface.
Etching the lens itself to provide diffusing risers adds significantly to the cost of the lens because each individual lens must be processed separately. Attempts at etching the dies used to form stamped fresnel lenses have reduced stage glare to some degree, but that approach is not compatible with all types of dies used to form lenses. That is particularly true where the materials used to form the dies are not susceptible to etching or, when etched, do not provide a surface that has the desired roughness or diffusing properties.
Thus, a need exists for a process of providing diffuse risers on a fresnel lens die.